It is known that an internal combustion engine, such as a Diesel engine or a gasoline engine, generally includes a cylinder block defining at least one cylinder that accommodates a piston coupled to rotate a crankshaft. The cylinder is closed by a cylinder head that cooperates with the piston to define a combustion chamber. A fuel and air mixture is periodically drawn or injected into and ignited in the combustion chamber, thereby resulting in hot exhaust gases whose expansion causes the reciprocating movement of the piston and thus the rotation of the crankshaft.
The fuel is generally provided by at least a fuel injector, which may be located directly into the combustion chamber and which may receive the fuel from a fuel rail in communication with a fuel tank through a fuel pump. The air is usually drawn into the combustion chamber through one or two intake valves, which selectively open and close a communication between the combustion chamber and an intake manifold. Likewise, the exhaust gases are usually discharged from the combustion chamber through one or more exhaust valves, which selectively open and close a communication between the combustion chamber and an exhaust manifold.
The intake and the exhaust valves are conventionally actuated by means of one or more overhead camshafts, which are located over the cylinder head. More specifically, the camshafts may be located directly within the cylinder head or in a Ladder Frame or a Cam Carrier fastened to the cylinder head. Some internal combustion engines, typically the internal combustion engines having only two valves per cylinder (i.e. one intake valve and one exhaust valve), are manufactured according to a Single Over Head Camshaft (SOHC) design, wherein a single camshaft is located over the cylinder head to actuate both the intake valves and the exhaust valves, traditionally via bucket tappets or intermediary rocker arms. Other internal combustion engines, typically the internal combustion engines having four or more valves per cylinder (e.g. at least two intake valves and two exhaust valves), are manufactured according to a Double Over Head Camshaft (DOHC) design, wherein two separated camshafts are located over the cylinder head to actuate the intake valves and the exhaust valves.
Independently from the specific design of the engine, the camshaft(s) are generally rotated by the engine crankshaft through a mechanical transmission, usually referred as timing drive, which synchronizes the rotation of the crankshaft and the camshaft(s), so that the engine valves open and close at proper times during each cylinder's intake and exhaust strokes. By way of example, a typical timing drive for a SOHC includes a first sprocket keyed on the crankshaft, a second sprocket keyed on the camshaft and a chain (or a toothed belt) wound around the first and the second sprocket. A typical timing drive for a DOHC includes the same components of the SOHC system, with the addition of two meshing gears that are individually keyed on a respective of the two camshafts to transmit torque from one another. In other embodiments, the two camshafts may be symmetrically coupled by means of a transmission chain or a transmission belt.
In order to guarantee the operation of the internal combustion engine, it is very important that the speed ratio between the crankshaft and the camshaft(s) is equal to a half, namely that the rotational speed of the crankshaft is two times the rotational speed of the camshaft(s), so that the engine valves open once every two complete rotations of the crankshaft.
To meet this requirement, the standard timing drives delineated above need that the diameter of the second sprocket (i.e. the one keyed on the camshaft) must be double the diameter of the first sprocket (i.e. the one keyed on the crankshaft). As a consequence, the second sprocket is generally a cumbersome component that, being located next to the cylinder head, may have the side effect of increasing the overall height of the internal combustion engine.
Another drawback of the known timing drives is that they are not easily interchangeable. In other words, an internal combustion engine manufactured according to a SOHC design cannot be easily adapted or transformed into an engine implementing a DOHC design and vice versa. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.